Abstract

The methylotrophic yeastPichia pastoriswas examined for functional expression of bovine opsin. An expression plasmid was constructed where the bovine opsin gene was placed downstream from theP. pastorisalcohol oxidase 1 gene promoter and fused at its amino-terminus to the acid phosphatase secretion signal. Quantitative–competitive PCR analysis of a stable yeast transformant showed that one copy of the opsin gene was integrated into the yeast genome. The expression level in this transformant corresponded to ∼0.3 mg of opsin per liter of cell culture (A600= 1.0). Sucrose density sedimentation analysis indicated that the opsin was associated exclusively with the membrane fraction. Similar to retinal opsin,P. pastoris-expressed opsin migrated as a single band of ∼37 kDa on SDS–PAGE and showed high mannose N-glycosylation. A portion of the expressed opsin (∼4–15%) reacted with 11-cis-retinal to form the rhodopsin chromophore (λmax500 nm), and after purification showed ground and excited state spectral characteristics indistinguishable from those of the native pigment. Further, the metarhodopsin-II-mediated G-protein-activating potential of yeast expressed rhodopsin was similar to that of native rhodopsin. These results show thatP. pastoriscells have the capacity to functionally express bovine opsin.

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